1. Field of the Invention
The present invention relates to a charged particle beam exposure system used, for example, in a lithographic method.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of miniaturized structures, such as semiconductor elements, integrated circuits, liquid crystal elements, micro-patterned members and micro-mechanical components.
A lithographic process comprises a plurality of lithographic steps in which patterns or structures to be formed on a substrate are imaged onto the substrate to expose a radiation sensitive layer provided on the substrate. The radiation sensitive layer, which is commonly referred to as a resist, may be exposed by optical radiation, such as visible or ultraviolet light, or by charged particles, such as ions or electrons. In the imaging of patterns using charged particles, a conventional method uses a plurality of charged particle beams or beamlets for writing the pattern onto the resist, wherein the beams or beamlets can be selectively switched on and off while the substrate carrying the resist is moved relative to the array of switchable beams.
The switchable beams are controlled by a deflector plate having a plurality of apertures traversed by the beams. Each aperture has a deflector associated therewith for selectively deflecting the beam traversing the aperture by a sufficient angle such that the beam will not reach the substrate carrying the resist. Such type of multi-aperture plate is also referred to as a blanking aperture plate (BAA) in the art. Background information on charged particle beam exposure systems using a plurality of charged particle beams controlled by a blanking aperture array may be obtained from US 2003/0025088 A1, the contents of which are incorporated herein by reference.
It is desirable to obtain a high through-put in terms of number of wafers exposed per hour. This number is limited by an available charged particle beam current for exposing locations on the wafer with an exposure dose above a threshold of the resist. However, the maximum current per exposing beam is limited due to Coulomb interactions and other effects deteriorating a maximum resolution of an exposed pattern.
A conventional system disclosed in U.S. Pat. No. 5,144,142 and the article of Berry et al., J. Vac. Sci. Technol. B 15.6., November/December 1997 0734-211X/97/15.6./2382/5/$10.00 ©1997 American Vacuum Society, pages 2382 to 2386, uses massively parallel beams for exposing each location on the wafer to achieve a necessary exposure dose with a relatively low current per individual beam. A plurality of apertures is disposed along a line on the blanking aperture plate, and the deflectors associated with the apertures are driven by outputs of stages of a shift register. An exposure pattern supplied to an input of the shift register is shifted in accordance with a clock signal, resulting in that a pattern of exposing beams will be translated across an image plane of the system. The wafer is moved in synchronism with the translating pattern such that each location on the wafer to be exposed receives a number of exposures from individual beams corresponding to the number of apertures in the line connected to the shift register. Other locations on the wafers which are not to be exposed will not receive any exposure dose.
It has been found that the conventional system using the apertures disposed in a line and controlled by stages of a shift register provide limited flexibility in providing exposure patterns suitable for manufacturing desired structures in the lithographic process.